Prospects for measuring rainfall using propagation differential phase in X- and K(a)-radar bands

Abstract

Model calculations and measurements of the specific propagation and backscatter differential phase shifts (KDP and δo, respectively) in rain are discussed for X- (λ to approximately 3 cm) and Ka-band (λ to approximately 0.8 cm) radar wavelengths. The details of the drop size distribution have only a small effect on the relationships between KDP and rainfall rate R. These relationships, however, are subject to significant variations due to the assumed model of the drop aspect ratio as a function of their size. The backscatter differential phase shift at X band for rain rates of less than about 15 mm h-1 is generally small and should not pose a serious problem when estimating KDP from the total phase difference at range intervals of several kilometers. The main advantage of using X-band wavelengths compared to S-band (λ to approximately 10-11 cm) wavelengths is an increase in KDP by a factor of about 3 for the same rainfall rate. The relative contribution of the backscatter differential phase to the total phase difference at Ka band is significantly larger than at X band. This makes propagation and backscatter phase shift contributions comparable for most practical cases and poses difficulties in estimating rainfall rate from Ka-band measurements of the differential phase. Experimental studies of rain using X-band differential phase measurements were conducted near Boulder. Colorado, in a stratiform, intermittent rain with a rate averaging about 4-5 mm h-1. The differential phase shift approach proved to be effective for such modest rains, and finer spatial resolutions were possible in comparison to those achieved with similar measurements at longer wavelengths. A KDP-R relation derived for the mean drop aspect ratio (R = 20.5KDP0.80) provided a satisfactory agreement between rain accumulations derived from radar measurements of the differential phase and data from several nearby high-resolution surface rain gauges. For two rainfall events, radar estimates based on the assumed mean drop aspect ratio were, on average, quite close to the gauge measurements with about 38% relative standard deviation of radar data from the gauge data.